In small, fractional horsepower, permanent magnet direct current motors a commutator typically comprises a number of copper segments laid on a cylindrical support. The segments are spaced apart by a small amount, so that there is a gap between adjacent segments. As a brush in contact with the commutator segment wears down, particles of carbon become embedded in the gaps. As the motor rotates these may be thrown off centrifugally. However it has been found that when using a low viscosity oil for lubricating bearings in such motors, the oil migrates from the bearing on to the commutator segments. This oil then mixes with the carbon particles to hold them in the gaps between the segments, which eventually produces an electrical short between the segments causing a breakdown or loss in efficiency of the motor. This may happen in particular when using low viscosity oils suitable for low temperature applications, for example automobiles.
It has been known to provide washers on the motor shaft between the bearing and commutator to throw the oil radially away from the shaft but these are not entirely satisfactory. GB777490 shows a system in which oil is thrown radially outwards by a cup-shaped collar into a circumferential collector mounted fast on the motor housing, the oil draining back to the bearing, but this requires the addition of several parts where space is at a premium in small motors.